Method and apparatus for measuring the common-mode component of a differential signal

Abstract

A common-mode detector includes a first difference amplifier that is connected to compare a first input voltage with a feedback voltage to provide a first result, a second difference amplifier that is connected to compare a second input voltage with the feedback voltage to provide a second result, and a feedback amplifier that is connected to drive the feedback voltage to a level that is substantially the average of the first and second input voltages in response to receiving the first and second results.

1. A common-mode detector, comprising:
 a first difference amplifier that is connected to compare a first input voltage with a feedback voltage to provide first and second differential current signals; 
 a second difference amplifier that is connected to compare a second input voltage with said feedback voltage to provide third and fourth differential current signals; 
 a feedback amplifier having inverting and non-inverting inputs, said feedback amplifier connected to receive said first and fourth differential current signals at said inverting input and said second and third differential currents at said non-inverting input to drive said feedback voltage to a level that is substantially the average of said first and second input voltages in response to receiving said first, second, third and fourth differential current signals. 
 
   
   
     2. The detector of  claim 1 , wherein said first and second difference amplifiers comprise:
 first and second differential transconductance amplifiers, respectively. 
 
   
   
     3. The detector of  claim 2 , wherein said first transconductance amplifier comprises:
 first and second outputs coupled to said inverting and non-inverting inputs, respectively, so that said first output provides current I P  and said second output provides current I N  according to
     I   P   =I   O /2+α( V   P   −V   N ) 
     I   N   =I   O /2−α( V   P   −V   N ) 
 
 
     wherein I O  is a bias current, V P  is said first input voltage, V N  is said feedback voltage, α is a gain coefficient of said first differential transconductance amplifier and I P  and I N  comprise said first and second differential current signals, respectively. 
   
   
     4. A common-mode detector, comprising:
 an op-amp having inverting and non-inverting inputs and a detector output; 
 first and second differential transconductance amplifiers, said differential transconductance amplifiers each having:
 a first input; 
 a second input coupled to said detector output; 
 a first output coupled to said inverting input; and 
 a second output coupled to said non-inverting input; 
 
 wherein, in response to receiving a differential signal at said first inputs, said common-mode detector is operable to provide a common-mode detector output indicative of a common-mode component of said differential signal. 
 
   
   
     5. The detector of  claim 4 , wherein said first differential transconductance amplifier further comprises:
 first and second transistors each having a collector coupled to said inverting and non-inverting inputs, respectively. 
 
   
   
     6. The detector of  claim 5 , further comprising:
 a first current source coupled to emitters of said first and second transistors. 
 
   
   
     7. The detector of  claim 6 , further comprising:
 an emitter degeneration resistor coupled between said current source and each of said emitters. 
 
   
   
     8. The detector of  claim 4 , further comprising:
 a first impedance coupled between a reference voltage and each of said inverting input and first outputs. 
 
   
   
     9. The detector of  claim 8 , further comprising:
 a second impedance coupled between said reference voltage and each of said noninverting input and said second outputs. 
 
   
   
     10. The detector of  claim 9 , wherein said first and second impedances comprise first and second resistors, respectively. 
   
   
     11. The detector of  claim 4 , wherein said first differential transconductance amplifier further comprises:
 first and second transistors with their output collectors coupled to said first and second outputs, respectively, and their bases to said first and second inputs, respectively; 
 so that said first differential transconductance amplifier is operable to receive said detector output and a first side of said differential signal. 
 
   
   
     12. The detector of  claim 11 , wherein said second differential transconductance amplifier further comprises:
 third and fourth transistors with their output collectors coupled to said second and first outputs, respectively, and their bases to said second and first inputs, respectively; 
 so that said second differential transconductance amplifier is operable to receive said detector output and a second side of said differential signal. 
 
   
   
     13. The detector of  claim 12 , further comprising:
 a current source coupled to emitters of said third and fourth transistors. 
 
   
   
     14. A method of measuring a common-mode component of a differential signal, comprising:
 converting a voltage differential between a non-inverting side of said differential signal and a feedback signal to generate a first differential current signal; 
 converting a voltage differential between an inverting side of said differential signal and said feedback signal to generate a second differential current signal; and 
 comparing said first and second differential current signals to generate said feedback signal, so that said feedback signal settles to a voltage indicative of the common-mode component of said differential signal. 
 
   
   
     15. The method of  claim 14 , wherein said comparing step comprises generating first and second voltages corresponding to said first and second differential current signals, and comparing said voltages to generate said feedback signal.